This review critically examines the effects of photo-oxidation on micro- and nanoplastics (MNPs) in aquatic and terrestrial environments. Photo-oxidation, primarily driven by sunlight, alters the physical, chemical, and biological properties of MNPs, influencing their adsorption of contaminants, colloidal stability, transport behavior, and toxicity to organisms. The process involves the generation of reactive radicals and the formation of environmentally persistent free radicals (EPFRs), leading to changes in surface properties such as increased O-containing functional groups and decreased hydrophobicity. These modifications affect the adsorption of organic pollutants (OPs) and heavy metals (HMs), enhancing or reducing their mobility in water and porous media. Photo-oxidation also influences the aggregation behavior of MNPs, with monovalent cations generally increasing stability and divalent cations having mixed effects. Natural colloids, such as humic acid (HA) and fulvic acid (FA), can either inhibit or promote photo-oxidation, depending on their interaction with MNPs. Additionally, MNPs can mediate the phototransformation of OPs and HMs, sometimes enhancing degradation and sometimes accelerating reduction. The biological effects of photo-oxidized MNPs include increased toxicity to aquatic organisms, altered biofilm formation, and changes in microbial communities. The review highlights the complex interplay between MNPs and environmental constituents, emphasizing the need for further research to understand the underlying mechanisms and the impact on ecosystem health.This review critically examines the effects of photo-oxidation on micro- and nanoplastics (MNPs) in aquatic and terrestrial environments. Photo-oxidation, primarily driven by sunlight, alters the physical, chemical, and biological properties of MNPs, influencing their adsorption of contaminants, colloidal stability, transport behavior, and toxicity to organisms. The process involves the generation of reactive radicals and the formation of environmentally persistent free radicals (EPFRs), leading to changes in surface properties such as increased O-containing functional groups and decreased hydrophobicity. These modifications affect the adsorption of organic pollutants (OPs) and heavy metals (HMs), enhancing or reducing their mobility in water and porous media. Photo-oxidation also influences the aggregation behavior of MNPs, with monovalent cations generally increasing stability and divalent cations having mixed effects. Natural colloids, such as humic acid (HA) and fulvic acid (FA), can either inhibit or promote photo-oxidation, depending on their interaction with MNPs. Additionally, MNPs can mediate the phototransformation of OPs and HMs, sometimes enhancing degradation and sometimes accelerating reduction. The biological effects of photo-oxidized MNPs include increased toxicity to aquatic organisms, altered biofilm formation, and changes in microbial communities. The review highlights the complex interplay between MNPs and environmental constituents, emphasizing the need for further research to understand the underlying mechanisms and the impact on ecosystem health.